[0001] The subject invention relates to an electronic control device according to the preamble
of claim 1.
[0002] In the construction of electronic control devices it is common practice to use a
printed wiring board which has an insulated surface that in turn supports a plurality
of conductive surfaces as one element of the device's structure. The printed wiring
board supports a number of electronic components that are interconnected by the printed
wiring board conductive areas into an operable device. This type of structure, if
exposed to a moist atmosphere, can fail due to condensation shorting out electrical
paths between the conductive areas. In electronic control devices that have no safety
function, this is an inconvenience, but not a safety problem. In devices that have
safety functions, the shorting out of the conductive areas by moisture can create
an unsafe mode of operation.
[0003] A typical type of control device that provides a safety function is a burner control
apparatus in which the device is responsible for the safe ignition of a fuel, with
subsequent monitoring of a flame. Typical of this type of device is an S89C Hot Surface
Ignition Control as manufactured by Honeywell Inc. The S89C utilizes a hot surface
ignitor which, when energized, ignites a gaseous fuel from a burner. The hot surface
ignitor then acts as a flame rod to provide a flame rectification signal that monitors
the presence of a flame at the burner. These types of units, if exposed to moisture,
can have condensation on their printed wiring boards which short out circuitry that
can cause the burner to issue forth fuel when no flame in fact exists. This type of
problem has been encountered in installations in food processing locations. It is
quite common in these environments for the control device to be exposed to atmospheric
moisture or to the spray of water from clean up activities.
[0004] This type of problem can be readily solved by providing completely sealed electronic
control devices, or by "potting" the device. Potting is a term used to generically
refer to electronic equipment which has been filled with a plastic-like material that
either becomes hard or semi-hard, but totally seals the device from moisture. The
use of a totally water-tight enclosure or potting creates both a cost penalty, and
a situation that makes repair of the device difficult or impossible.
[0005] It is therefore the object of the subject invention to devise an electronic control
circuit which shuts down safely if atmospheric moisture or spray of water is encountered
in its environment. This object is achieved according to the characterizing features
of claim 1. Further advantageous embodiments of the control circuit may be taken from
the sub-claims.
[0006] The present invention is directed to an arrangement for providing an electronic control
device with a dew sensor that disables the electronic control device in a safe manner.
The dew sensor of the present invention allows for a normal mode of operation when
moisture is not present within the device, but causes an alternate safe mode of operation
in the event that an excessive amount of moisture is present within the device to
an extent where it has condensed on the conductive areas of the printed wiring board
to such a degree as to cause a leakage path between the conductive areas. The present
invention is implemented in the form of a dew sensor which utilizes adjacent edges
of two conductive areas of the printed wiring board pattern. The dew sensor structure
is connected to the gate of a field effect transistor through a resistor, and is capable
of driving the gate of the field effect transistor so as to cause the field effect
transistor to shut down the electronic control device in the alternate safe mode of
operation whenever moisture collects within the device due to condensation.
[0007] The present invention can be implemented by providing more than one dew sensor structure
within a device so that more than one electronic control means jean be operated to
the alternate safe mode of operation. The present invention allows for the addition
of a dew sensing function merely by providing an appropriate conductive pattern of
the printed wiring board conductive areas, and the addition of a single resistor in
the gate of the electronic control means or field effect transistor. The addition
of the single resistor, and the organization of the conductive areas to provide the
dew sensing function, adds little cost to the device.
[0008] The dew sensing function of the present invention is significantly different than
the prior art dew sensing arrangements wherein discrete moisture sensitive elements
are placed either external to a device, or internal to a device, to sense the presence
of moisture. Moisture sensors have been used in alarm circuits, window controls, and
controls to raise and lower blinds or tops on automobiles. These types of devices
utilize discrete separate sensors and circuitry that is dedicated to the moisture
control function. Also, dew sensors have been used as discrete elements within video
tape recorders to disable the tape drive mechanisms in the event that moisture condensed
in the unit.
[0009] Under reference to the attached drawings an embodiment of the inventive control device
shall be described, where:
Figure 1 is a block diagram of a typical hot surface ignition control;
Figure 2 is a detailed circuit diagram of a device disclosed in Figure 1 with the
invention also incorporated; and
Figure 3 is a partial disclosure of a printed wiring board of the device of Figure
2.
[0010] In Figure 1 there is disclosed at 10 an electronic control device that could be a
form of the previously mentioned S89C Hot Surface Ignition Control as manufactured
by Honeywell Inc. The control device 10 is connected by terminals 11 and 12 to an
appropriate source of power and is operated in response to a thermostat 13. Contained
within the control device 10 is a warmup timer 14 that allows for a sufficient period
of warmup of a hot surface ignitor element 15. The warmup timer 14 controls a transistor
16 that in turn is connected in series with a relay 1K. The relay 1K has a pair of
relay contacts lKl and 2Kl that function as part of a safe start check and warmup
function. Their detailed operation will not be explained as it is not directly pertinent
to the invention, except to state that the state of the relay 1K determines in part
whether the control device 10 can function safely.
[0011] The control device 10 further has a sensor circuit 20 that is connected to a pair
of voltage terminals 21 and 22 which are in turn connected through relay contacts
1K3 and 2K3 to the hot surface ignitor 15. It is obvious that when the relay contacts
1K3 and 2K3 are closed, the hot surface ignitior 15 is connected to the voltage terminals
21 and 22 and can heat. If the contacts 1K3 and 2K3 are open circuited, as shown in
the drawing, the hot surface ignitor 15 is connected by a conductor 23 to the sensor
circuit 20, and acts as a flame rod. This function is a function contained in the
S89C mentioned before. The sensor circuit 20 (along with a timing circuit 30) has
an output transistor 24 that is connected in series with a relay 2K which has a pair
of contacts lK2 and 2K2. The lK2 contact is a normally closed contact that acts as
part of a safe start check circuit along with the relay contact lKl. The contact 2K2
is used to energize a circuit to a gas valve or fuel valve disclosed at 25. Since
the relay 2K controls the contact 2K2 that connects the fuel valve 25 to a source
of power, it is quite apparent that the operation of the transistor 24 in energizing
or de-energizing the relay 2K effectively controls the fuel valve 25.
[0012] The control device 10 is generally completed by the timing circuit 30 in turn having
an output transistor 31 and an associated relay 3K. The relay3K controls the two contacts
1K3 and 2K3 that energize and de-energize the hot surface ignitor 15.
[0013] In normal operation of the S89C type device, the closing of the thermostat 13 causes
the warmup timer 14 to allow the hot surface ignitor 15 to become energized for a
set period of time. A safe start check is run which includes the relay contacts lKl
and lK2. After a sufficient warmup period of time has occurred, the timing circuit
30 operates the relay 2K to energize the contact 2K2 and open the gas valve or fuel
valve 25. The hot surface ignitor 15 ignites the fuel and is ready to act as a flame
rectification sensor. The timing circuit 30 then causes the relay contacts lK3 and
2K3 to open circuit which de-energizes the hot surface ignitor as an ignition element,
and leaves it solely as a flame rectification device. The circuit operation just described
basically is the type of circuit operation of the S89C of the Honeywell Hot Surface
Ignition Control device. This type of device relies on a very small flame rectification
signal on the conductor 23, and the device is being improved to avoid any possible
problem with moisture. The device has been disclosed as a prior art device. The present
invention is incorporated in this type of a prior art device, and will now be described
in connection with the circuit of Figure 2.
[0014] In Figure 2 an electronic control device 10' is disclosed. It incorporates the present
novel dew sensing arrangement. Only small portions of the circuit diagram will be
specifically identified and will correspond with the circuit diagram of Figure 1.
Only the specific areas of the novel dew sensor means will be explained.
[0015] The energizing terminals 11 and 12 have been disclosed as supplying power to the
control device 10'. Within this device is a warmup timer 14 that has an output control
transistor 16 in series with the relay 1K. Within the warmup time of timer 14 is a
field effect transistor 40 that has its source to drain connection between a ground
conductor 41 and a source of potential conductor 42. The field effect transistor has
a control gate 43 that in turn is connected through a resistor 44 to a dew sensor
means generally disclosed at 45. The dew sensor means 45 includes a conductor 46 and
a parallel conductor 47 that form part of a printed wiring board pattern, as will
be disclosed in connection with Figure 3. As long as no moisture is present at the
dew sensor means 45, there is no conduction between the conductive surfaces 46 and
47, and the gate 43 of the field effect transistor 40 is effected solely by the signal
supplied through a resistor 50 from other parts of the warmup timer 14.
[0016] If moisture becomes a problem, a collection of moisture across conductors 46 and
47 of the dew sensor means 45 causes a resistive path to exist between the gate 43
of the field effect transistor 40, the resistor 44, and a junction 51 of a potential
source for the device. The sensor means 45 is effective when used with a high impedance
switch means. As soon as conduction occurs due to moisture at the dew sensor means
45, the gate 43 of the field effect transistor 40 is clamped to a potential that causes
the field effect transistor 40 to be driven full off thereby stopping the current
drive into the base for the transistor 16. This in turn overrides the normal control
of the transistor 16, and in turn overrides the normal control of relay 1K. This locks
the relay 1
K in a safe condition so that the balance of the circuitry cannot allow fuel to flow
through the fuel valve disclosed at 25.
[0017] The electronic control device 10' further has a sensor circuit 20 that corrsponds
to the sensor circuit 20 oi Figure 1. The output control transistor 24, and the control
relay 2K is again shown. It will be noted that the transistor 24 is dependent on the
conductive state of a field effect transistor 60 that has a gate 61. It is obvious
that when the field effect transistor 60 is conducting, the conduction effectively
shorts out or grounds the base of transistor 24 thereby controlling its operation.
A second dew sensing means 62 is disclosed as being madeup in part by the conductor
63 and the parallel conductor 64 that form part of the printed wiring board pattern,
as will be shown in Figure 3. The conductor 64 is connected through a resistor 65
to the gate 61 of the field effect transistor 60. Once again, if dew is present at
the dew sensor means 62, a conductive path will be established through the resistor
65, the conductor 64, and the conductor 63 to the ground conductor 41. This effectively
grounds the gate 60 of the field effect transistor 60 and overrides the control of
the transistor 24 to cause the relay 2K to be de-energized thereby opening the contact
2K2 which removes power to the fuel valve 25.
[0018] The control device 10' is completed by a timing circuit 30 similar to the timing
circuit disclosed in Figure 1. The timing circuit 30 has an output transistor 31 that
in turn controls the relay 3K. This timing circuit has been identified merely to provide
identification so that a block diagram of Figure 1 can be correlated with the complete
diagram of Figure 2.
[0019] No attempt has been made to describe all of the circuit components of Figure 2 and
their associated functions. Since the circuitry of Figure 2 basically is contained
in the prior art device of Figure 1, except for the dew sensing means portions, only
the dew sensing means portions have been-described in any detail. In Figure 3, the
actual implementation of the dew sensing means structure on the printed wiring board
will be described.
[0020] In Figure 3 a portion of a printed wiring board means 70 is disclosed. The printed
wiring board means 70 is of conventional sturcture having an insulated surface 71
upon which is placed a plurality of conductive areas 72. The conductive areas 72 have
been identified as dark, patterned arrangements. Between the conductive areas, the
general distribution of electronic components have been shown. For example, a capacitive
element is shown at 73, a diode has been shown at 74 and a resistor has been shown
at 75. No attempt is going to be made to identify all of the printed wiring board
conductive area pattern. Only two areas, those areas which make up the dew sensor
means, will be specifically identified. The dew sensor means 45 is identified by the
two generally parallel conductive surface areas 46 and 47. It will be noted that the
parallel edges of the conductive surfaces 46 and 47 create a small spaced gap 76 that
can be readily seen in Figure 2 at the dew sensor means 45. It is apparent that in
normal operation the gap 76 is insulated, but will change to a variable impedance
or resistance if moisture condenses on the printed wiring board surface at 76. A conductive
path caused by moisture between the elements 46 and 47, as previously explained, causes
the field effect transistor 40 to operate in a safe mode for the device.
[0021] The second dew sensing means 62 is disclosed where the conductor 63, which forms
part of the ground circuit for the device lies generally parallel to the conductive
area 64. A gap 77 is created by the conductive areas 63 and 64, and this gap forms
part of the dew sensing means 62 as disclosed in Figure 2. Once again, as long as
no moisture is present on the printed wiring board surface, the circuit between the
conductor 63 and 64 is an open circuit. If moisture condenses in the gap 77, the dew
sensor means 62 causes the gate 61 of the field effect transistor 60 to take over
the control of the device via the transistor 24 and the relay 2K.
[0022] It is apparent that as many dew sensing means as are desired can be fabricated into
a device by creating one or more pairs of parallel conductive areas that are in turn
connected to the control elements of electronic control means, such as field effect
transistors. The present disclosure shows two dew sensing means which are each individually
capable of controlling the device in the event that moisture enters the control device
10'. It is quite apparent that one such dew sensing means could be used, or that more
than two could be used by properly orienting conductive areas on the printed wiring
board means and connecting these areas to the gates of the field effect transistors
or to the electrodes of the individual transistors used elsewhere in the circuit.
1. An electronic control device (10) , including:
printed wiring board means (70) that has an insulated surface (71) that in turn supports
a plurality of conductive areas (72); and
electronic components (R, CR, C) mounted by said printed wiring board means (70),
and electrically interconnected by said conductive areas (72) to provide an electronic
circuit (14) for said control device (10), characterized in that said electronic components
including at least one electronic control means (40) having a plurality of electrodes
(41, 42, 43) connected to said conductive areas as part of said control device with
said electronic control means having a plurality of states of conductivity; and
said conductive areas including at least one pair of spaced edges (46, 47) to form
dew sensing means (45); and said pair of spaced edges (46, 47) of said dew sensing
means (45) being capable of connecting between one of said plurality of electronic
control means electrodes (43) and said electronic circuit (14), with said dew sensing
means (45) changing impedance between said spaced edges (46, 47) with the presence
of vapor at said dew sensing means to in turn cause said electronic control means
(40) to change said state of conductivity to cause said control device to change from
a normal mode of operation to an alternate mode of operation.
2. Control device according to claim 1, characterized in that said electronic control
components include a plurality of electronic control means (40; 60) each having a
plurality of electrodes (43; 61) connected to said conductive areas as part of said
control device, and with said electronic control means each having a plurality of
states of conductivity; said conductive areas including a plurality of pairs of spaced
edges (46, 47; 63, 64) with each pair of said spaced edges forming a dew sensor (45;
62); and each one of said electronic control means (40; 60) being connected to one
of said dew sensors; said electronic control means conducting between said plurality
of states of conductivity depending on the presence or absence of moisture at said
dew sensors; moisture at said dew sensors causing said control device to change from
said normal mode of operation to said alternate safe mode of operation.
3. Control device according to claim 1 or 2, characterized in that said spaced edges
(46, 47; 63, 64) are generally parallel spaced edges.
4. Control device according to claim 1 or 2, characterized in that said electronic
control means is a field effect transistor (40; 60); and said change in impedance
of said dew sensing means (45; 62) in response to moisture is a change in resistance.
5. Control device according to claim 1 or 2, characterized in that said dew sensing
means (45; 62) includes a resistor (44; 64) in series circuit with said one of said
plurality of electrodes (43; 61) connected to said dew sensing means (45; 62).
6. Control device according to claim 4, characterized in that said dew sensing means
(45; 62) change in impedance is a change in resistance.